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<H1><a name="Preprocessor">10 Preprocessing</a></H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Preprocessor_nn2">File inclusion</a>
<li><a href="#Preprocessor_nn3">File imports</a>
<li><a href="#Preprocessor_condition_compilation">Conditional Compilation</a>
<li><a href="#Preprocessor_nn5">Macro Expansion</a>
<li><a href="#Preprocessor_nn6">SWIG Macros</a>
<li><a href="#Preprocessor_nn7">C99 and GNU Extensions</a>
<li><a href="#Preprocessor_delimiters">Preprocessing and delimiters</a>
<ul>
<li><a href="#Preprocessor_nn8">Preprocessing and %{ ... %} &amp; " ... " delimiters</a>
<li><a href="#Preprocessor_nn9">Preprocessing and { ... } delimiters</a>
</ul>
<li><a href="#Preprocessor_typemap_delimiters">Preprocessor and Typemaps</a>
<li><a href="#Preprocessor_nn10">Viewing preprocessor output</a>
<li><a href="#Preprocessor_warning_error">The #error and #warning directives</a>
</ul>
</div>
<!-- INDEX -->



<p>
SWIG includes its own enhanced version of the C preprocessor.  The preprocessor
supports the standard preprocessor directives and macro expansion rules.
However, a number of modifications and enhancements have been made.  This
chapter describes some of these modifications.
</p>

<H2><a name="Preprocessor_nn2">10.1 File inclusion</a></H2>


<p>
To include another file into a SWIG interface, use the <tt>%include</tt> directive
like this:
</p>

<div class="code">
<pre>
%include "pointer.i"
</pre>
</div>

<p>
Unlike, <tt>#include</tt>, <tt>%include</tt> includes each file once (and will not
reload the file on subsequent <tt>%include</tt> declarations).  Therefore, it
is not necessary to use include-guards in SWIG interfaces.
</p>

<p>
By default, the <tt>#include</tt> is ignored unless you run SWIG with the
<tt>-includeall</tt> option.   The reason for ignoring traditional includes
is that you often don't want SWIG to try and wrap everything included
in standard header system headers and auxiliary files.

<H2><a name="Preprocessor_nn3">10.2 File imports</a></H2>


<p>
SWIG provides another file inclusion directive with the <tt>%import</tt> directive.
For example:
</p>

<div class="code">
<pre>
%import "foo.i"
</pre>
</div>

<p>
The purpose of <tt>%import</tt> is to collect certain information from another 
SWIG interface file or a header file without actually generating any wrapper code.
Such information generally includes type declarations (e.g., <tt>typedef</tt>) as well as
C++ classes that might be used as base-classes for class declarations in the interface.
The use of <tt>%import</tt> is also important when SWIG is used to generate
extensions as a collection of related modules.   This is an advanced topic and is described
in later in the <a href="Modules.html#Modules">Working with Modules</a> chapter.
</p>

<P>
The <tt>-importall</tt> directive tells SWIG to follow all <tt>#include</tt> statements
as imports.    This might be useful if you want to extract type definitions from system 
header files without generating any wrappers.

<H2><a name="Preprocessor_condition_compilation">10.3 Conditional Compilation</a></H2>


<p>
SWIG fully supports the use of <tt>#if</tt>, <tt>#ifdef</tt>,
<tt>#ifndef</tt>, <tt>#else</tt>, <tt>#endif</tt> to conditionally
include parts of an interface.  The following symbols are predefined
by SWIG when it is parsing the interface:
</p>

<div class="code"><pre>
SWIG                            Always defined when SWIG is processing a file
SWIGIMPORTED                    Defined when SWIG is importing a file with <tt>%import</tt>
SWIG_VERSION                    Hexadecimal (binary-coded decimal) number containing SWIG version,
                                such as 0x010311 (corresponding to SWIG-1.3.11).

SWIGCSHARP                      Defined when using C#
SWIGGUILE                       Defined when using Guile
SWIGJAVA                        Defined when using Java
SWIGJAVASCRIPT                  Defined when using Javascript
SWIG_JAVASCRIPT_JSC             Defined when using Javascript for JavascriptCore
SWIG_JAVASCRIPT_V8              Defined when using Javascript for v8 or node.js 
SWIGLUA                         Defined when using Lua
SWIGMZSCHEME                    Defined when using Mzscheme
SWIGOCAML                       Defined when using OCaml
SWIGOCTAVE                      Defined when using Octave
SWIGPERL                        Defined when using Perl
SWIGPHP                         Defined when using PHP (any version)
SWIGPHP7                        Defined when using PHP7
SWIGPYTHON                      Defined when using Python
SWIGR                           Defined when using R
SWIGRUBY                        Defined when using Ruby
SWIGSCILAB                      Defined when using Scilab
SWIGTCL                         Defined when using Tcl
SWIGXML                         Defined when using XML
</pre></div>

<p>
In addition, SWIG defines the following set of standard C/C++ macros:
</p>

<div class="code">
<pre>
__LINE__                        Current line number
__FILE__                        Current file name
__STDC__                        Defined to indicate ISO C
__cplusplus                     Defined when -c++ option used
</pre>
</div>

<p>
Interface files can look at these symbols as necessary to change the
way in which an interface is generated or to mix SWIG directives with
C code. These symbols are also defined within the C code generated by
SWIG (except for the symbol `<tt>SWIG</tt>' which is only defined
within the SWIG compiler).
</p>

<H2><a name="Preprocessor_nn5">10.4 Macro Expansion</a></H2>


<p>
Traditional preprocessor macros can be used in SWIG interfaces.  Be aware that the <tt>#define</tt> statement
is also used to try and detect constants.  Therefore, if you have something like this in your file,
</p>

<div class="code">
<pre>
#ifndef _FOO_H 1
#define _FOO_H 1
...
#endif
</pre>
</div>

<p>
you may get some extra constants such as <tt>_FOO_H</tt> showing up in the scripting interface.
</p>

<p>
More complex macros can be defined in the standard way. For example:
</p>

<div class="code">
<pre>
#define EXTERN extern
#ifdef __STDC__
#define ISOC_(args)   (args)
#else
#define ISOC_(args) ()
#endif
</pre>
</div>

<p>
The following operators can appear in macro definitions:
</p>

<ul>
<li><tt>#x</tt><br>
Converts macro argument <tt>x</tt> to a string surrounded by double quotes ("x").
</li>

<li><tt>x ## y</tt><br>
Concatenates x and y together to form <tt>xy</tt>.
</li>

<li><tt>`x`</tt><br>
If <tt>x</tt> is a string surrounded by double quotes, do nothing.  Otherwise, turn into a string
like <tt>#x</tt>.  This is a non-standard SWIG extension.
</li>
</ul>

<H2><a name="Preprocessor_nn6">10.5 SWIG Macros</a></H2>


<p>
SWIG provides an enhanced macro capability with the <tt>%define</tt> and <tt>%enddef</tt> directives. 
For example:
</p>

<div class="code">
<pre>
%define ARRAYHELPER(type, name)
%inline %{
type *new_ ## name (int nitems) {
  return (type *) malloc(sizeof(type)*nitems);
}
void delete_ ## name(type *t) {
  free(t);
}
type name ## _get(type *t, int index) {
  return t[index];
}
void name ## _set(type *t, int index, type val) {
  t[index] = val;
}
%}
%enddef

ARRAYHELPER(int, IntArray)
ARRAYHELPER(double, DoubleArray)
</pre>
</div>

<p>
The primary purpose of <tt>%define</tt> is to define large macros of code.  Unlike normal C preprocessor
macros, it is not necessary to terminate each line with a continuation character (\)--the macro definition
extends to the first occurrence of <tt>%enddef</tt>.    Furthermore, when such macros are expanded,
they are reparsed through the C preprocessor.  Thus, SWIG macros can contain all other preprocessor
directives except for nested <tt>%define</tt> statements.
</p>

<p>
The SWIG macro capability is a very quick and easy way to generate large amounts of code.  In fact,
many of SWIG's advanced features and libraries are built using this mechanism (such as C++ template
support).
</p>

<H2><a name="Preprocessor_nn7">10.6 C99 and GNU Extensions</a></H2>


<p>
SWIG-1.3.12 and newer releases support variadic preprocessor macros.  For example:
</p>

<div class="code">
<pre>
#define DEBUGF(fmt, ...)   fprintf(stderr, fmt, __VA_ARGS__)
</pre>
</div>

<p>
When used, any extra arguments to <tt>...</tt> are placed into the
special variable <tt>__VA_ARGS__</tt>.   This also works with special SWIG
macros defined using <tt>%define</tt>.
</p>

<p>
SWIG allows a variable number of arguments to be empty.  However, this often results
in an extra comma (, ) and syntax error in the resulting expansion. For example:
</p>

<div class="code">
<pre>
DEBUGF("hello");   --&gt; fprintf(stderr, "hello", );
</pre>
</div>

<p>
To get rid of the extra comma, use <tt>##</tt> like this:
</p>

<div class="code">
<pre>
#define DEBUGF(fmt, ...)   fprintf(stderr, fmt, ##__VA_ARGS__)
</pre>
</div>

<p>
SWIG also supports GNU-style variadic macros.    For example:
</p>

<div class="code">
<pre>
#define DEBUGF(fmt, args...)  fprintf(stdout, fmt, args)
</pre>
</div>

<p>
<b>Comment:</b> It's not entirely clear how variadic macros might be useful to
interface building.   However, they are used internally to implement a number of
SWIG directives and are provided to make SWIG more compatible with C99 code.
</p>

<H2><a name="Preprocessor_delimiters">10.7 Preprocessing and delimiters</a></H2>


<p>
The preprocessor handles { }, " " and %{ %} delimiters differently.
</p>

<H3><a name="Preprocessor_nn8">10.7.1 Preprocessing and %{ ... %} &amp; " ... " delimiters</a></H3>


<p>
The SWIG preprocessor does not process any text enclosed in a code block %{ ... %}.  Therefore,
if you write code like this,
</p>

<div class="code">
<pre>
%{
#ifdef NEED_BLAH
int blah() {
  ...
}
#endif
%}
</pre>
</div>

<p>
the contents of the <tt>%{ ... %}</tt> block are copied without
modification to the output (including all preprocessor directives).
</p>

<H3><a name="Preprocessor_nn9">10.7.2 Preprocessing and { ... } delimiters</a></H3>


<p>
SWIG always runs the preprocessor on text appearing inside <tt>{ ... }</tt>.  However,
sometimes it is desirable to make a preprocessor directive pass through to the output
file.  For example:
</p>

<div class="code">
<pre>
%extend Foo {
  void bar() {
    #ifdef DEBUG
      printf("I'm in bar\n");
    #endif
  }
}
</pre>
</div>

<p>
By default, SWIG will interpret the <tt>#ifdef DEBUG</tt> statement.   However, if you really wanted that code
to actually go into the wrapper file, prefix the preprocessor directives with <tt>%</tt> like this:
</p>

<div class="code">
<pre>
%extend Foo {
  void bar() {
    %#ifdef DEBUG
      printf("I'm in bar\n");
    %#endif
  }
}
</pre>
</div>

<p>
SWIG will strip the extra <tt>%</tt> and leave the preprocessor directive in the code.
</p>

<H2><a name="Preprocessor_typemap_delimiters">10.8 Preprocessor and Typemaps</a></H2>


<p>
<a href="Typemaps.html#Typemaps">Typemaps</a> support a special attribute called <tt>noblock</tt> where the { ... } delimiters can be used,
but the delimiters are not actually generated into the code.
The effect is then similar to using "" or %{ %} delimiters but the code <b>is</b> run through the preprocessor. For example:
</p>

<div class="code">
<pre>
#define SWIG_macro(CAST) (CAST)$input
%typemap(in) Int {$1= SWIG_macro(int);}
</pre>
</div>

<p>
might generate
</p>

<div class="code">
<pre>
  {
    arg1=(int)jarg1;
  }
</pre>
</div>

<p>
whereas
</p>

<div class="code">
<pre>
#define SWIG_macro(CAST) (CAST)$input
%typemap(in, noblock=1) Int {$1= SWIG_macro(int);}
</pre>
</div>

<p>
might generate
</p>

<div class="code">
<pre>
  arg1=(int)jarg1;
</pre>
</div>

<p>
and
</p>

<div class="code">
<pre>
#define SWIG_macro(CAST) (CAST)$input
%typemap(in) Int %{$1=SWIG_macro(int);%}
</pre>
</div>

<p>
would generate
</p>

<div class="code">
<pre>
  arg1=SWIG_macro(int);
</pre>
</div>


<H2><a name="Preprocessor_nn10">10.9 Viewing preprocessor output</a></H2>


<p>
Like many compilers, SWIG supports a <tt>-E</tt> command line option to display the output from the preprocessor.
When the <tt>-E</tt> option is used, SWIG will not generate any wrappers.
Instead the results after the preprocessor has run are displayed.
This might be useful as an aid to debugging and viewing the results of macro expansions.
</p>

<H2><a name="Preprocessor_warning_error">10.10 The #error and #warning directives</a></H2>


<p>
SWIG supports the commonly used <tt>#warning</tt> and <tt>#error</tt> preprocessor directives.
The <tt>#warning</tt> directive will cause SWIG to issue a warning then continue processing.
The <tt>#error</tt> directive will cause SWIG to exit with a fatal error.
Example usage:
</p>

<div class="code">
<pre>
#error "This is a fatal error message"
#warning "This is a warning message"
</pre>
</div>

<p>
The <tt>#error</tt> behaviour can be made to work like <tt>#warning</tt> if the <tt>-cpperraswarn</tt>
commandline option is used. Alternatively, the <tt>#pragma</tt> directive can be used to the same effect, for example:
</p>

<div class="code">
<pre>
  /* Modified behaviour: #error does not cause SWIG to exit with error */
  #pragma SWIG cpperraswarn=1
  /* Normal behaviour: #error does cause SWIG to exit with error */
  #pragma SWIG cpperraswarn=0
</pre>
</div>

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